(Last PR today I promise! Sorry for all the noise)

This PR fixes a few issues I ran into in putting together the other two:

• It provides a clear error when a test dependency is missing or fails. In my case, PHP wasn't installed, but previously all I saw was every PHP test failing and:

  Uncaught AssertionError: expected SyntaxError { message: 'Unexpected end of JSON input' } to be null
  

  Now, if PHP isn't installed, you get:

  Uncaught AssertionError: Process 'php' exited unexpectedly with code 127
  

• It removes the Travis config that's doing nothing here (because Travis is no longer doing free open source builds) and replaces it with an equivalent GitHub Actions config that works
• It updates the coverage scripts so that they work too. The only thing required to make this upload to Codeclimate correctly is to get the test reporter id (Codeclimate project -> Repo Settings -> Test Coverage -> Test reporter id) and set it as a secret called CODECLIMATE_REPO_TOKEN in this repo.

You can see a passing test build of this in the actions on my repo here: https://github.com/httptoolkit/httpsnippet/actions/runs/1555694270